Thermal transfer systems have been developed to obtain prints from pictures that have been generated electronically, for example, from a color video camera or digital camera. An electronic picture can be subjected to color separation by color filters. The respective color-separated images can be converted into electrical signals. These signals can be operated on to produce individual electrical signals corresponding to certain colors, for example, cyan, magenta, or yellow. These signals can be transmitted to a thermal printer. To obtain a print, a colored dye-donor layer, for example black, cyan, magenta, or yellow, can be placed face-to-face with a dye image-receiving layer of a receiver element to form a print assembly that can be inserted between a thermal print head and a platen roller. The thermal print head can be used to apply heat from the back of the dye-donor. The thermal print head can be heated sequentially in response to the various electrical signals, and the process can be repeated as needed to print all desired colors. A color hard copy corresponding to the original picture can be obtained. A laminate layer can be provided over the color image. Further details of this process and an apparatus for carrying it out are set forth in U.S. Pat. No. 4,621,271 to Brownstein.
At the high temperatures used for thermal dye transfer, for example, about 150° C. to about 200° C., many polymers used in thermal printing ribbons can soften, causing wrinkling of the ribbon, resulting in unwanted lines in the transferred image. A wrinkle can form near the border area of an image. For example, it can spread or extend from a trailing or rear portion of a used dye transfer area at least to a leading or front portion of the next dye transfer area to be used. As a result, a crease or wrinkle can form in the leading or front portion of the next dye transfer area to be used, causing an undesirable line artifact to be printed on a corresponding section of a leading or front portion of the dye receiver when dye transfer occurs at the crease. The line artifact printed on the dye receiver can be relatively short, but quite visible. In fast thermal printing, because of the higher temperature and/or faster movement of the printing ribbon, wrinkling becomes more of a concern.
Various methods of reducing wrinkle formation in the final image are known. For example, mechanical mechanisms that stretch the thermal printing ribbon during printing to prevent crease or wrinkle formation are disclosed in U.S. patent applications Ser. Nos. 10/394888 and 10/392502. JP 1999-024368 discloses the use of organic resin fine particles and silicone particles in a dye-donor layer of a thermal printing ribbon to improve the release of a dye from the dye-donor layer to a receiver, reducing sticking of the donor and receiver, and thereby reducing wrinkle formation. However, these methods do not directly address some fundamental factors that can affect wrinkling, i.e., the physical properties of the thermal printing ribbon. U.S. Pat. No. 6,475,696 discloses the use of inorganic particles such as nanoparticles to increase the stiffness of receiver supports for photographic elements, for example, photographic films and papers. The increased stiffness provides desired handling properties for the finished photographic product, but does not reduce the appearance of wrinkles in the image because the wrinkles are generated by the thermal printing ribbon.
JP 1999-208079 and corresponding EP 0909659 disclose a reusable donor for resistive head thermal printing, wherein the donor ribbon substrate includes a low thermally conductive polymer matrix and high thermally conductive metal particles. The particles are oriented such that the long axis of the particles corresponds to the thickness of the substrate. One or more particles can be used to span the thickness of the support. According to the disclosure, the magnetic particles are included to increase the efficiency of heat transfer to the dye-donor element, to increase the thickness and/or strength of the donor support, and to reduce slippage of the support. No effect on wrinkling is described.
A means of eliminating or reducing the formation of creases or wrinkles in a thermal printing ribbon that does not have the problems associated with the prior art is desired. It is further desired that the thermal printing ribbon have desirable bending stiffness, thickness, thermal conductivity, and thermal dimensional stability to help in controlling wrinkle or crease. It is further desired that such a ribbon be capable of high speed printing.